Single DB query tree generation with awesome_nested_set

application_helper.rb

module ApplicationHelper
  # Mixin for tree generation based on a nested set, just place this in your application_helper.rb
  module TreeMethods
    attr_accessor :child_nodes
    attr_accessor :parent_node
    def after_initialize
      @child_nodes = []
      @parent_node = nil
    end
    def recursive_tree nodes=nil, level=0
      if @child_nodes.nil?
        after_initialize
      end
      nodes = self_and_descendants if nodes.nil?
      nodes.shift unless nodes.first.parent_node.nil?
      nodes.each do |node|
        if node.id!=id && node.parent_id == id
          node.extend TreeMethods
          @child_nodes << node
          node.recursive_tree nodes, level+1
        end
      end
      self
    end
  end

  def nested_tree base
    unless base.nil?
      if base.is_a?(ActiveRecord::Base) && base.respond_to?(:self_and_descendants)
        base.extend TreeMethods;base.recursive_tree 
      elsif base.is_a?(Class) && base.respond_to?(:acts_as_nested_set)
        base.roots.collect{|c| c.extend TreeMethods;c.recursive_tree} unless base.roots.nil?
      end
    end
  end  
end

view.erb

render :partial=>category.child_nodes

your_controller.rb

@categories = nested_tree Category # calling with Class
@categories = nested_tree Category.root # calling with Instance

# or just try it on the console:
helper.nested_tree Category